A conventional Bragg grating comprises an optical fiber in which the index of refraction undergoes periodic perturbations along its length. The perturbations may be equally spaced in the case of an unchirped grating, or may be unequally spaced in the case of a chirped grating. The grating reflects light over a given waveband centered around a wavelength equal to twice the spacing between successive perturbations. The remaining wavelengths pass essentially unimpeded. Such Bragg gratings are typically employed in a variety of applications including filtering, stabilization of semiconductor lasers, reflection of fiber amplifier pump energy, and compensation for fiber dispersion.
Fiber Bragg gratings are also important components in optical communication systems that employ wavelength-division multiplexing. In such systems, it is important that the carrier wavelength of each channel is maintained at a precise value, which is typically about +/-0.1 nm. Unfortunately, both the refractive index of the grating and the distance between successive perturbations are temperature dependent. As a result, the reflected waveband is also temperature dependent. In many cases, however, it is desirable to provide a stabilized reflection band that is temperature independent. U.S. Pat. No. 5,042,898 (Morey et al.) discloses a temperature-independent Bragg grating in which wavelength changes resulting from changes in strain are used to compensate for wavelength changes resulting from variations in the temperature of the grating. In particular, a constant wavelength of reflected light may be maintained during a drop in temperature by increasing the longitudinal strain on the fiber. This reference uses a complex mechanical arrangement of materials with differing, but positive, coefficients of thermal expansion (CTE). Specifically, in this reference a portion of the optical fiber containing the grating is sectioned off by securing the optical fiber at each side of the grating to separate metallic compensating members arranged for longitudinal movement relative to one another. The CTEs of the two compensating members are both positive and different from one another. By mechanically adjusting the compensating members longitudinally relative to each other to thereby vary the distance between them, there is imposed on the optical grating a longitudinal strain of a magnitude that varies to balance out wavelength variations resulting from changes in the temperature of the grating. This known temperature compensating package, however, is complex and expensive to manufacture.
The same result achieved by the compensating package disclosed in the previously mentioned patent can be achieved with a less complex arrangement by fabricating the package from a material that has a negative coefficient of thermal expansion (CTE). When such a material is employed, the reflected wavelength of the fiber grating will be substantially independent of temperature if the package has a CTE equal to the percent change in wavelength per degree Celsius of the uncompensated fiber grating. For typical applications, the package must be formed from a material having a CTE in range of -14.5.times.10.sup.-6 to -8.times.10.sup.-6 /.degree. C.
Materials having a negative CTE are generally either difficult to produce or relatively expensive. Moreover, it is particularly difficult to provide a negative CTE material that precisely compensates for temperature variations without any overcompensation or undercompensation. Accordingly, it would be desirable to provide a thermal compensating package for a fiber Bragg grating that has an appropriate CTE so that the reflection wavelength of the grating is substantially temperature independent. This, and other needs, are met by the present invention, as hereinafter described.